The present invention relates to high strength friction stir welding and, more particularly, relates to reducing material property degradation of friction stir weld joints during subsequent heat treatments.
Friction stir welding is utilized to join workpieces to form structural assemblies that can be used in the manufacture of military and commercial aircraft, as well as in other applications requiring high strength weld joints. As illustrated in FIG. 1, friction stir welding involves inserting the threaded pin 10a of a rotating friction stir welding tool 10 between the opposing faces of a pair of workpieces 12, 14 while urging the workpieces together. Friction stir welding can also be used to repair cracks or other defects in a single workpiece. The rotation of the threaded pin 10a between the opposing faces of the workpieces 12, 14, or within a single workpiece, creates friction that generates sufficient heat energy to plasticize the workpiece material in the weld zone 16. The friction stir welding tool 10 also includes a concave shoulder adapted to consolidate the plasticized workpiece material within the weld zone 16 as the friction stir welding tool is moved along the interface 11 between workpieces or through a single workpiece. A friction stir weld joint 18 forms, joining the workpieces together in a unitary assembly, as the plasticized regions of the workpieces 12, 14 flow together and cool in the weld zone 16. See U.S. Pat. No. 5,460,317 to Thomas et al. for a general discussion of friction stir welding, the entire contents of which are incorporated herein by reference.
One particular benefit of friction stir welding is that the formation of the weld joint 18 is autogenous and is created by the solidification of the plasticized parent materials rather than a filler material, as is commonly used in conventional welding processes. In addition, as illustrated in FIG. 2A, the friction stir weld joint 18 comprises a nugget having a refined grain structure with grains having an equiaxed shape and grain sizes ranging in order of magnitude from approximately 0.0001 to 0.0002 inches (approximately 3 to 5 microns). As a result of the improved grain structure, the friction stir weld joint 18 resists the formation and propagation of micro-cracks and exhibits improved strength, ductility and toughness, as well as improved corrosion and fatigue resistance.
The frictional heat necessary to plasticize the workpiece material during friction stir welding can degrade the material properties of the parent materials. As shown in FIG. 1, during friction stir welding, the frictional heat created by the rotating friction stir welding tool 10 is conducted from the weld zone 16 through the workpieces 12, 14 into the ambient environment, creating a heat-affected region 20 around the weld zone 16. The elevated temperatures associated with the friction stir welding process can degrade the material properties of the parent materials, including the strength, stiffness, and ductility of the workpieces 12, 14.
Material property degradation is particularly problematic when friction stir welding precipitation hardened parent materials, which have improved mechanical properties obtained through solution and precipitation heat treatments. When friction stir welding precipitation hardened workpieces 12, 14, the joined workpieces commonly require additional precipitation hardening or a resolution heat treatment to recover the parent material properties. The resolution heat treatment includes solution heat treating the workpieces 12, 14 at a predetermined temperature schedule and then rapidly cooling the workpieces by quenching. The solution heat treating process is then followed by a precipitation heat treatment involving either natural or artificial aging at a second predetermined temperature schedule to recover the parent material properties. While resolution heat treating improves the material properties of the joined workpieces 12, 14, the resolution heat treatment typically results in appreciable grain growth in the friction stir weld joint 18, as illustrated by a comparison of FIGS. 2A and 2B. For example, friction stir weld joints 18 commonly have grain sizes of up to 0.25 inches after the resolution heat treatment. The large grains in the friction stir weld joint 18 resulting from the resolution heat treatment adversely affect the material properties of the weld joint, including reducing the hardness, ductility, resistance to intergranular corrosion, and fatigue resistance.
In seeking to minimize the degradation of the material properties of friction stir weld joints 18 during post weld heat treatments, several alternative approaches have been proposed, including shortening the duration of the solution heat treatment, post-weld annealing prior to solution heat treatment, and surface peening. However, these approaches have not been effective in reducing the grain growth of friction stir weld joints 18 during post-weld solution heat treatments.
Thus, there is a need for improved methods and apparatus for friction stir welding heat treated materials and, in particular, precipitation hardened materials. Such manufacturing methods and apparatus should realize the improved material properties associated with resolution heat treating while minimizing degradation of the material properties of the friction stir weld joint during such heat treatments.
The present invention provides an improved precipitation hardened structural assembly formed by friction stir welding and a method and apparatus of forming the same. According to one embodiment of the present invention, a precipitation hardened structural assembly is provided, including a first structural member and a second structural member positioned adjacent to the first structural member such that the first and second structural members define an interface therebetween. At least one friction stir weld joint joins the first structural member to the second structural member at least partially along the interface. The first and second structural members and the friction stir weld joint are solution heat treated at a first predetermined temperature schedule and precipitation heat treated at a second predetermined temperature schedule and wherein the friction stir weld joint comprises a refined grain structure having a grain size of less than about 5 microns. In one embodiment, the first and second structural members comprise dissimilar materials. In another embodiment, at least one of the first and second structural members is formed from aluminum, aluminum alloys, titanium, or titanium alloys.
The present invention also provides an apparatus for attachment to a rotatable spindle for forming a friction stir weld joint. In one embodiment, according to the present invention, the apparatus includes a friction stir welding tool in rotatable communication with the spindle. The friction stir welding tool defines a cavity. The apparatus includes at least one heater adapted to thermally communicate with the friction stir welding tool to thereby heat the tool and wherein the at least one heater is at least partially received in the cavity of the friction stir welding tool. The at least one heater can include a resistance heating coil, an induction heating coil, a quartz lamp, a gas torch, or a laser. In one embodiment, the at least one heater thermally communicates with the friction stir welding tool through convection, conduction, irradiation or induction. In another embodiment, the apparatus includes a sensor in thermal communication with the friction stir welding tool for measuring the temperature of the friction stir welding tool. In yet another embodiment, the apparatus includes a controller in electrical communication with the sensor and in operable communication with the at least one heater. The controller is configured to automatically modify the heat output of the at least one heater to modify the temperature of the friction stir welding tool.
In another embodiment, the present invention provides an apparatus for friction stir welding at least one structural member, including a machine having a rotatable spindle. A friction stir welding tool is in rotatable communication with the spindle. The apparatus includes at least one heater adapted to thermally communicate with the friction stir welding tool to thereby heat the tool and wherein the at least one heater is structured so as to be electrically insulated from the at least one structural member. The at least one heater can include a resistance heating coil, an induction heating coil, a quartz lamp, a gas torch, or a laser. In one embodiment, the friction stir welding tool defines a cavity adapted to at least partially receive the at least one heater. In another embodiment, the at least one heater is spaced from the friction stir welding tool. In another embodiment, the at least one heater thermally communicates with the friction stir welding tool through convection, conduction, irradiation or induction. In yet another embodiment, the apparatus includes a sensor in thermal communication with the friction stir welding tool for measuring the temperature of the friction stir welding tool. In still another embodiment, the apparatus includes a controller in electrical communication with the sensor and in operable communication with the at least one heater. The controller is configured to automatically modify the heat output of the at least one heater to modify the temperature of the friction stir welding tool.
The present invention also provides a method of forming a friction stir weld joint, including mounting a friction stir welding tool to a rotatable spindle such that the friction stir welding tool rotates with the spindle. The friction stir welding tool is heated with at least one heater to thereby inhibit grain growth in the weld joint. Subsequent to the heating step, the friction stir welding tool is inserted into at least one structural member. The friction stir welding tool is moved through the at least one structural member to form the friction stir weld joint. In one embodiment, the method includes heating the friction stir welding tool concurrently with the inserting step. According to another embodiment, the at least one structural member is precipitation hardened prior to the inserting step. In yet another embodiment, the at least one structural member and friction stir weld joint are solution heat treated at a predetermined temperature schedule subsequent to the moving step. Thereafter, the at least one structural member and friction stir weld joint are precipitation heat treated by aging at a second predetermined temperature schedule. According to another embodiment, the heating step comprises transferring heat to the friction stir welding tool through convection, conduction, irradiation, or induction. In another embodiment, the heating step comprises heating the friction stir welding tool to a temperature between about 600xc2x0 F. and about 1000xc2x0 F. In yet another embodiment, the method includes measuring the temperature of the friction stir welding tool. In still another embodiment, the method includes automatically modifying the heat output of the at least one heater to thereby modify the temperature of the friction stir welding tool.
The present invention also provides a method of manufacturing a structural assembly, including forming a friction stir weld joint in at least one structural member using a rotating friction stir welding tool. The method includes heating the friction stir weld tool prior to and during the forming step with at least one heater to thereby inhibit grain growth in the weld joint. In one embodiment, the forming and heating steps are repeated to thereby join at least one additional structural member to the structural assembly. In another embodiment, the at least one structural member is precipitation hardened at a predetermined temperature schedule prior to the forming step. In still another embodiment, the structural assembly is solution heat treated at a predetermined temperature schedule subsequent to the forming step. Thereafter, the structural assembly is precipitation heat treated by aging at a second predetermined temperature schedule. According to another embodiment, the heating step comprises transferring heat to the friction stir welding tool through convection, conduction, irradiation, or induction. In another embodiment, the heating step comprises heating the friction stir welding tool to a temperature between about 600xc2x0 F. and about 1000xc2x0 F. In yet another embodiment, the method includes measuring the temperature of the friction stir welding tool. In still another embodiment, the method includes automatically modifying the heat output of the at least one heater to thereby modify the temperature of the friction stir welding tool.
Accordingly, the present invention provides an improved precipitation hardened structural assembly having one or more friction stir weld joints with refined grain structure and a method and apparatus for constructing the same. The method and apparatus for constructing the structural assembly minimize degradation of the material properties of the friction stir weld joint during subsequent resolution heat treatments thereby effectively realizing the improved material properties associated with both friction stir welding and precipitation hardening.